1. Field of the Invention
The present invention concerns a method for determining flip angle distributions for various antenna transmission configurations in at least one specific volume region within an examination subject in a magnetic resonance system, wherein the magnetic resonance system has a radio-frequency antenna with a number of resonator elements that (in different transmission configurations) can be excited individually or in groups for generation of linearly-independent radio-frequency field distributions in an examination volume enclosing the examination subject. The invention also concerns a magnetic resonance system suitable for implementation of such a method, with a corresponding radio-frequency antenna and a computer program product that can be loaded into a memory of a programmable control device of such a magnetic resonance system for implementation of the method.
2. Description of the Prior Art
Magnetic resonance tomography, also called magnetic resonance tomography apparatus, is a widespread technique for acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method, the body or a body part of the patient to be examined must initially be exposed to an optimally homogeneous static basic magnetic field (usually designated as a B0 field) that is generated by a basic field magnet of the magnetic resonance system. During the acquisition of the magnetic resonance images, rapidly-switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatial coding. Moreover, radio-frequency pulses of a defined field strength are radiated into the examination subject with radio-frequency antennas. The magnetic flux density of these radio-frequency pulses is typically designated with B1. The pulse-shaped radio-frequency field is therefore generally called a B1 field. The nuclear spins of the atoms in the examination subject are excited by means of these radio-frequency pulses such that they are deflected from their equilibrium state (parallel to the basic magnetic field B0) by what is known as an “excitation flip angle” (generally also called a “flip angle”). The nuclear spins then process around the direction of the basic magnetic field B0. The magnetic resonance signals thereby generated are acquired by radio-frequency reception antennas. The reception antennas can be either the same antennas with which the radio-frequency pulses are also radiated, or separate reception antennas. The magnetic resonance images of the examination subject are generated on the basis of the acquired magnetic resonance signals. Each image point in the magnetic resonance image is thereby associated with a small body volume (known as a “voxel”) and each brightness or intensity value of the image points is linked with the signal amplitude of the magnetic resonance signal acquired from this voxel. The correlation between a resonant radiated radio-frequency pulse with the field strength B1 and the flip angle α achieved thereby is provided by the equation
                              α          =                                    ∫                              t                =                0                            τ                        ⁢                          γ              ·                                                B                  1                                ⁡                                  (                  t                  )                                            ·                                                          ⁢                              ⅆ                t                                                    ,                            (        1        )            wherein γ is the gyromagnetic ratio which can be considered as a fixed material constant for most magnetic resonance examinations, and τ is the effective duration of the radio-frequency pulse. Aside from being dependent on the duration of the pulse, the flip angle achieved by the emitted radio-frequency pulse (and thus the strength of the magnetic resonance signal) also depends on the strength of the radiated B1 field. Spatial fluctuations in the field strength of the excited B1 field therefore lead to unwanted variations in the acquired magnetic resonance signal that can adulterate the measurement result.
                              α          =                                    ∫                              t                =                0                            τ                        ⁢                          γ              ·                                                B                  1                                ⁡                                  (                  t                  )                                            ·                                                          ⁢                              ⅆ                t                                                    ,                            (        1        )            
Especially at high magnetic field strengths (that are inevitably present due to the necessitated basic magnetic field B0 in a magnetic resonance tomography apparatus) the radio-frequency pulses have a non-homogeneous penetration behavior in conductive and dielectric media such as, for example, tissue. This leads to the situation that the B1 field can vary significantly within the measurement volume. In particular in the ultra-high field range with magnetic field strengths B0≧3 T, significant influences of the radio-frequency penetration behavior on the image quality are observed. Due to the B1 focusing and shielding effects, the flip angle of the radio-frequency pulses is a function of the location. Contrast and brightness of the acquired magnetic resonance images therewith vary in the mapped tissue and can lead, in the worst case, to the situation in which pathological structures are not visible.
Multi-channel transmission coils, also called “transmit arrays”, are presently considered as a promising approach to the solution of this problem. These are radio-frequency antennas of the aforementioned type that have a number of resonator elements that can be activated individually or in groups. This is possible, for example, when the individual resonator elements are electromagnetically decoupled from one another, and can be separately activated with an individual amplitude and phase via separate radio-frequency channels. Depending on with which amplitudes and phases the different transmission configurations (i.e. the individual resonator elements or groups of resonator elements or transmission modes) are excited, different radio-frequency distributions form in the examination volume of the antenna. For example, with an antenna with N resonator elements that are electromagnetically decoupled from one another and that can be activated individually, it is possible to transmit in N linearly-independent transmission modes which respectively lead to different field distributions.
A simple example of this is a birdcage resonator that has rods that are activated individually with regard to amplitude and phase. Each of these rods generates a B1 field independent of one another, with the B1 fields of the individual rods overlapping to form the total field distribution. Instead of individually considering the single resonator rods, different “collective excitation modes” of the entire birdcage antenna can also be individually excited. One of these modes is, for example, the standard excitation mode of the birdcage resonator (known as the “CP mode”) in which the radio-frequency voltage from rod-to-rod varies in terms of the phase by 360°/NR (wherein NR is the number of the rods). For higher order collective modes, the voltage then varies, for example, by 2·360°/NR, 3·360°/NR, etc. from rod to rod. For activation of such collective modes, a resident-power mode matrix is installed, for example, in the hardware used for the activation of the antenna elements.
Through individual settings of the amplitude and the phase of the radio-frequency pulse radiated by each transmission configuration (i.e. by each transmission element or each transmission mode), the spatial distribution of the B1 field can be influenced with the goal to generate an optimally homogeneous radio-frequency field in the subject or, respectively, in the examination volume. Magnetic resonance systems of this type are, for example, specified in U.S. Pat. No. 6,043,658 and DE 10 2004 045 691 A1.
A significant, and as of yet largely unresolved, step in this context is the determination of the optimal individual transmission parameters, i.e. of the transmission amplitudes and transmission phases for the individual transmission configurations. Previous approaches for determination of optimized transmission parameters have been based either on simulations, such as in DE 10 2004 045 691 A1. For this purpose, a body model is required in order to simulate the radio-frequency penetration behavior and to be able to calculate the required parameters. A subject-specific optimization (i.e. an optimization that is directly adapted to the current examination subject), however, is therewith not possible. Alternatively, the absolute B1 distribution of every single transmission element can be measured per slice. This does in fact allow an object-dependent adjustment, but these measurements are extraordinarily time-consuming. Approximately 10 sec. per transmission configuration and per slice acquisition are presently required. For six transmission configurations and 10 slices, this leads to an adjustment time of 10 minutes. This method is thus not practical in practice as an adjustment method.